Dispersed throughout lung airway mucosa are individual and innervated groups of cells with unique morphological, cytochemical and biochemical properties similar to neuroendocrine cells (NE) in the G.I. tract, carotid body, taste bud, thyroid and pituitary. The presence of serotonin, amine precursor uptake and decarboxylation enzyme mechanism, acetyl cholinesterase, neuron specific enolase and at least one (bombesin) or more (enkephalins, calcitonin) neuropeptides suggests lung NE cells may have sensory or receptor mechanisms linked with respiratory centers in the CNS. These cells are difficult to locate in adult lung. However, 20 to 30% of all lung tumors are believed to be of NE cell origin possible in response to environmental influence such as cigarette smoke, ozone, halo ethers, hypoxic conditions and diethylnitrosomine. Furthermore it is proposed that these cells may play an important role in early lung development since they appear to be the first cells to differentiate and are most numerous during the embryonic and fetal periods. Our objectives are to carry out systematic morphological, cytochemical and biochemical studies of NE cells in the fetal period. In order to do so, new systems of analysis need to be developed.